User deleted from user groups

Hello ISPConfig mates,
I encountered some kind of bizarre behaviour on my server: some users form (let's say) webX use cvs on the same server. I've created a user group cvsusers and added all users created with ISPConfig to this user group with YAST (SUSE 9.3) The weird thing is that every time I create a new user or change some properties of the existing user through ISPConfig web page all the users I've added to cvsusers group are kicked out of this group and I have to add them again.
Is it normal? Can I do something to prevent this behaviour?